As volumes of Internet traffics are increased, optical communication networks having large capacity have been demanded. Therefore, a system having a signal transmission speed of 100 G (bit/s) or more per one wavelength has been developed.
However, when a signal transmission speed (bit rate) per one wavelength is increased, signal deterioration due to deterioration of OSNR (Optical Signal Noise Ratio) resistance and waveform distortion caused by a nonlinear effect such as wavelength dispersion or polarization mode dispersion is increased.
On the other hands, in recent years, a digital coherent receiver technology which is expected to improve the signal deterioration caused by the deterioration of the OSNR resistance or the waveform distortion has been attracting attention.
The coherent receiver technology is a reception technique which mixes a received optical signal and local oscillation light in a receiver with each other, extracting information on an electric field (a phase and an intensity of the light) of the received optical signal, converting the electric-field information of the light into an electric signal, and decodes the electric signal.
Furthermore, in the digital coherent receiver technology, an electric signal of the extracted electric-field information is quantized and converted into a digital signal by an A/D converter, and the digital signal is decoded by a digital signal process.
Use of the digital coherent receiver technology improves the OSNR resistance and waveform distortion resistance by performing compensation of the wavelength distortion through the digital signal process. Therefore, when compared with a general direct detection method, the digital coherent receiver technology may have excellent characteristics in terms of optical transmission in a large bit rate.
As the related art of the coherent reception, a technique of controlling a wavelength or a frequency of local oscillation light output from a local oscillator has been proposed (as for the related arts, refer to Japanese Laid-open Patent Publication Nos. 2010-109847 and 6-338856).
In the digital coherent receiver technology, control is performed such that a carrier frequency of a received optical signal and an oscillation frequency of local oscillation light oscillated by a local oscillator are made coincide with each other within a range of an allowable frequency difference.
Furthermore, the digital coherent receiver technique may be applied to a receiver included in an optical transmission system or the like which performs WDM (Wavelength Division Multiplexing) transmission by performing an Add (insertion)/Drop (branch) operation using arbitrary channels.
In such an optical transmission system, since the Add/Drop operation is performed using the wavelength channels, a wavelength of a transmitted optical signal, i.e., a carrier frequency is changed. In this case, if a difference between the carrier frequency of the optical signal and the oscillation frequency of the local oscillation light is within the range of the allowable frequency difference (pull-in range), both the frequencies are locked again and synchronized with each other. However, the frequency difference is out of the range of the allowable frequency difference, pull-in is not performed and loss of synchronization occurs, and accordingly, transmission quality is deteriorated.